LIVERWORTS 171 
and absorbs the necessary nourishment from it. When 
ripe, the spore-case is ruptured (Fig. 167, B), and the light 
spores are scattered; and when they germinate they pro- 
duce new thallus bodies. 
96. Alternation of generations.—The life-history of Mar- 
chantia shows a distinct alternation of generations, and 
since this is a feature of all the higher plants it is necessary 
to understand it clearly. The thallus body produces no 
spores, but produces sperms and eggs; that is, it produces 
gametes, and hence is called a gametophyte, which means a 
“gamete plant.” The gametes produce an odspore; but 
the odspore does not produce a new thallus plant, producing 
instead a spore-case. This structure, called the spore-case, 
does not produce gametes, but produces spores; and hence 
it is called a sporophyte, which means a “spore plant.” 
Thus in the life-history of AMfarchantia and of all higher 
plants, there is an alternation of gametophyte and sporo- 
phyte. It is evident that in this alternation of generations 
the gametophyte is the sexual and the sporophyte is the 
sexless generation. Therefore, odspores are produced by the 
gametophyte, and ordinary spores by the sporophyte; but 
the odspores always produce sporophytes, and the ordinary 
spores always produce gametophytes. These relations may 
be indicated clearly by the following formula, in which G 
and S are used for gametophyte and sporophyte respec- 
tively: 
G \ Oh oe J ee oe oe ee 
| —o/ | —o/ 
The formula indicates that the gametophyte produces 
two gametes, which fuse to form an oéspore, which produces 
the sporophyte, which produces an ordinary spore, which 
produces a gametophyte, etc. It will be remembered that a 
similar alternation of generations was noted in the red 
Alge (§ 75) and in the mildews (§ 82) among the Thallo- 
